This invention relates to hybrid systems and control methods in general and describes a two mode electro-mechanical transmission and its control in particular.
A power-train includes a power generator and a power transmission system. The internal combustion engine is the most common form of power generator. The power transmission system further includes a transmission, a differential and drive shafts. Engines have limited speed and torque ranges. Within these ranges, there are usually small regimes where engine can achieve optimal operation in terms of the lowest fuel consumption, the lowest emission or the highest performance. On the other hand, vehicle demands vary widely not only in power but also in torque and speeds. To facilitate discussion, a combination of torque and speed at the end of an input or output shaft is defined hereafter as a power state. A given power can be produced at or delivered with different combinations of torque and speed, and therefore has different power states. It can be seen that, engine and vehicle can have different power states. Matching these power states is the primary task of the transmission.
There are two types of transmissions commonly seen today on the marketplace, the stepwise and the step-less. The stepwise transmissions are further classified into manual transmissions and automatic transmissions. Through selecting appropriate gears for power transmission, they offer a limited number of gear ratios. Between two adjacent gear ratios, vehicle speed is regulated through change in engine speed.
Step-less transmissions are capable of providing, within a range, an infinite number of gear ratios. Vehicle speed can be fully regulated through continuous change of gear ratio without affecting engine speed. Thus engine can be operated at its optimal speed region. In addition, step-less transmissions offer other advantages over stepwise transmissions such as driving comfort, and are able to best utilize engine's maximum power over wide vehicle speeds. For this reason, step-less transmissions have received increase attention over the years. Two particular types of step-less transmissions, the toroidal traction drive and the belt function drive, have been successfully made to marketplace. The belt drives are the most popular among step-less transmissions.
The above mentioned conventional transmissions are not able to provide complete matches in power and in power states between the vehicle and engine. This is due to the fact that the conventional transmissions are designed to regulate speed or torque, but not power. The input power of the transmission is always equal to the output power assuming no internal power loss. In other words, the transmission can not regulate both output speed and torque independently.
The emerging concepts of hybrid power-trains, have opened a new avenue for power transmission and regulation. Among various concepts proposed, the series electric hybrid systems and parallel electric hybrid systems are the most simple and representative ones. The series hybrid system is the simplest power-train configuration, where the engine, generator, motor and driveline form a single series power path. The generator and motor set can be regarded as a step-less transmission. When used with energy storage devices, such as battery or capacitor, the transmission is able to regular output power in addition to regulating output speed. In this sense, the transmission is capable of controlling independently both output speed and torque.
In parallel hybrid system, there exist two independent parallel power paths from power sources to drive wheels. One is formed by the conventional mechanical drive line, referred to as the mechanical power path, the other is formed by a battery pack and an electric machine, referred to as the electric power path. The mechanical power path provides speed regulation as with the conventional power-train system, the electric power path provides power regulation or independent torque regulation. To achieve the best possible performance, it is desirable to use step-less transmission in the mechanical power path.
The advantages of series hybrid system include system simplicity and flexibility. Since full engine power has to pass through both generator and motor, the rating requirements for electric machines are high and system is big and heavy. As energy passing through the power train, it undergoes double conversions. Therefore, system efficiency is relatively low. The parallel hybrid system, on the other hand, sends only a portion of the power through electric power path, power ratings for electric machine is comparatively low and the system efficiency is high. Parallel hybrid system requires two independent power paths or power systems, subsequently inquires a higher cost penalty. Parallel hybrid systems are suited for mild hybrid systems.